FIG. 4 shows a fuel rod which is a nuclear fuel assembly used in, for example, a light water reactor. In the fuel rod, fuel pellets 3 such as UO.sub.2 are inserted into a cladding tube 2 made of zircaloy and are urged by a plenum spring 4, and the plugs 5 and 6 are welded at both ends of the cladding tube 2 by a TIG welding process.
Because of the TIG welding process to weld the plugs 5 and 6 to the cladding tube 2, heat affected zones with widths of approximately 2 mm are formed on both sides of the beads at the welded portions M. When the tube is quenched naturally after completion of the welding process, the heat affected zones are cooled down, in general, at a rate of approximately 100.degree. C./sec.
Before the welding process, the cladding tube 2 originally has a working-affected structure while the plugs 5 and 6 originally have structures of equiaxed grains. The TIG welding causes formation of the beads 7 and the heat affected zones 8 which have affected structures. The beads 7 have a fine needle-like grain structure caused by rapid quenching. The heat affected zones 8 have a structure of both equiaxed grains and needle-like grains, which is caused by rapid quenching from the high temperature at which a beta phase (b.c.c.) or both a beta phase and an alpha phase, which are stable zirconium phases at a relatively high temperature, are exhibited.
As the importance of nuclear power generation for a electric power supply sources have recently increased, improvement of the economical efficiency and operation efficiency of light water reactors have been increasingly demanded. To achieve these objects, the frequency of exchanging fuel rods must be reduced by enhancing the corrosion resistance of the fuel rods. A cladding tube of Zircaloy containing Nb and Fe, which improves the corrosion resistance as compared with prior Zircaloys, has been proposed. The improved Zircaloy is a zirconium alloy which has a composition of 0.6 to 2.0 weight percent Nb, 0.5 to 1.5 weight percent Sn, 0.05 to 0.3 weight percent Fe, and the remainder of Zr and unavoidable impurities.
The cladding tube 2 of the zirconium alloy containing Nb and Fe, disclosed in Japanese Patent Application, First Publication No. Hei 10-54891, improves the corrosion resistance as compared with prior cladding tubes containing the Zircaloy-2 (JIS H4751ZrNT802D) and Zircaloy-4(JIS H4751ZrNT804D).
When the cladding tube of the zirconium alloy containing Nb and Fe is welded in a TIG process, Nb and Fe are segregated at grain boundaries 11 of equiaxed grains 9 and needle-like grains 10 as shown in FIG. 5, so that at the grain boundaries 11 Nb is 3.5 weight percent and Fe is 0.6 weight percent.
At the heat affected zone 8 of the welded portion M, the cladding tube 2 of the zirconium alloy containing Nb and Fe may have inferior corrosion resistance to the cladding tube of Zircaloy-2 or Zircaloy-4.
To estimate the in-reactor corrosion rate, an autoclave test under a temperature of 360.degree. C. and a pressure of approximately 190 atmospheres is, in general, used.
In the autoclave test for the cladding tube 2 of zirconium alloy containing Nb and Fe, it has been found that the heat affected zones 8 of the bead 7 and the plug are coated with a black oxide film, while the heat affected zone 8 of the thin cladding tube is coated with a white oxide film which does not influence the corrosion resistance. The oxide films are subject to accelerated corrosion which leads to shortening their life.
The Japanese Patent Application, First Publication No. Hei 10-54891, teaches that, by cooling down heat affected zones 8 of the cladding tube of zirconium alloy containing Nb and Fe at a rate of 70.degree. C./sec. to 5.degree. C./sec. after the TIG welding process, the segregation of Nb and Fe at the grain boundaries 11 of the heat affected zone 8 is enhanced so that the heat affected zone 8 contains 4.0 to 30 weight percent Nb and 0.9 to 20 weight percent Fe, preventing occurrence of the white oxide film at the heat affected zone 8 and enhancing the corrosion resistance.
As a result of cooling the heat affected zone 8 at a temperature reduction rate exceeding 70.degree. C./sec., the heat affected zone 8 will contain Nb in amounts below 4 weight percent and Fe in amounts below 0.9 weight percent at the grain boundaries 11, and will have poor corrosion resistance. Even when the temperature reduction rate at the heat affected zone 8 is below 5.degree. C./sec., it is impossible to enhance the Nb content to above 30 weight percent and the Fe content to above 20 weight percent, which leads to a decrease in the strength of the fuel rod instead of an improvement of the corrosion resistance.
To adjust the temperature reduction rate to the range of 70.degree. C./sec. to 5.degree. C./sec. at the heat affected zone 8 after the TIG welding process, the following methods can be employed: (1) control to reduce the flow rate of a cooling gas provided to a welded portion M, (2) removing a chiller metal used to cool the welded portion M, and (3) a heat treatment step for heating the welded portion M by a heater after the welding step (using induction heating or directly applying a current). When methods (1) and (2) are employed, corrosion resistance cannot be improved because the temperature reduction rate remains high. In the method (3), corrosion resistance can be improved because the temperature is appropriately controlled, but installation of another device is needed, increasing the installation and running costs.